3 answers

I have seen similar 'delayed' times using upload on a LAN. It would seem to take upwards of 30 seconds before uploading of the first page began. Once the upload initiated, each page would take <1s to complete. If you upload a larger batch of say 20-30 images do you notice similar behaviour, i.e. the upload time between a 2 page and 20 page upload is not relative. This was using 9.0.0.3.

I've since tested on 9.0.1 over a WAN and did not see the initial 30 seconds delay. Perhaps something has changed in the latest release or an attribute of the LAN test network was to blame.

The only upload setting that may affect performance is one that can attempt to group files during upload, provided a minimum file size that is bigger than the largest file size of a scanned image, but it's not relevant here.

Thirty seconds is egregious and I would expect it's caused by some network impact. I would start by auditing the IIS logs, assuming you have relevant logging columns enabled, and see if there is any applicable time explanation.

Since the upload page uses the Daeja viewer, I have seen some delays as a result of initializing the viewer; especially with java versions 1.8.0-60 through 1.8.0-75 or so. Using those JRE versions can cause the viewer to take 30 seconds to a minute to initialize.